The most prolific weeds I feed them, would have to be chickweed (aptly named, of course). It only makes an appearance when there's moisture and the temperatures are low. The higher temps will send it to flower and escalate it's decline. Which doesn't make them as palatable to the chickens. Although, they'll still eat them, if its the only greens they have access to.
Chickweed will die off in the hotter months (late Spring to early Autumn) so while its prolific, its also temporary. I grab swathes of it, by the handful, and throw them into the coop.
Good old dandelion is a tasty treat too. It prefers the same conditions as chickweed, but is more palatable than chickweed, after setting flower. I pluck individual leafs where I can, and the chickens will break pieces off by shaking it in their beaks. Or they'll pull it apart between each other, when there's a tussle for ownership.
While dandelion will have a longer growing season than chickweed, it's not as prolific. I find my numbers of self-seeding dandelions are reducing every year, as I improve the soil. Dandelions love compacted soil, and are nature's tillers, with their large tap roots.
Milk thistle - Prickly Lettuce
If there was candy for chickens, this would be it. Milk thistle, not to be confused with the Scottish thistle, with large purple flowers. I call it milk thistle, since that's what my grandfather introduced to me, on his farm. He said the milky-white sap was a great cure for warts, and it was. In terms of chicken food, this is their absolute favourite. No wonder, as it's the closest relative of the cultivated lettuce.
More so than the dandelions, however, I've noticed they are becoming rarer in the garden. That's because the kangaroos and hares all think its herbivore candy too. Their favourite stage to eat them down, are young, like the image above. I'll pull the whole lot up by the roots, preferably when its bigger than in the image. The chickens peck at the leafs, and they break apart easily. They do prefer milk thistle before it flowers, but will still eat it, after it has.
The next weeds are a little more bitter, so the chickens will eat less. But I still pop them in, because its medicinal and adds variety. The above is Ribwort Plantain. Bitter weeds can help stave off worms and other nasties which can sometimes enter their stomachs. They just don't need a lot of it though. So an occasional food, not a daily one.
I pull the whole lot up by the roots, and throw into their coop. It makes it easier for the chickens to pull apart, when there's more mass.
Another of the bitter weeds I feed them occasionally, is dock. It looks a little like horseradish, and is often confused with it. Dock is a relative of the buckwheat family though.
Like ribwort, the leafs are tougher and bitter, so chickens won't delight on them as much. I notice the native herbivores will eat these down too, if the grass isn't growing. So its not really a weed I can rely on, most of the time. It's best eaten when young. Not easy to pull up by the roots, so I will pluck several leaves and give them to the chickens with a bunch of other weeds at the same time.
Cobbler's pegs - Farmer's friends
What I feed the chickens the most, because its prolific nearly all year round (and isn't too bitter) is good, old-fashioned cobbler's pegs. Also known as farmer's friends. I can feed them young and tender, like the swathe of new ones emerging, in the image above. Or I can pick them when they're bigger, and have gone to flower. They're very easy to pull up from the soil.
Flowering Cobbler's pegs
This image, is the cobbler's pegs, most people are familiar with. They have tiny yellow flowers, which turn into black, sticky seeds, that catch on your clothes as you brush past. I don't know why they're called "farmer's friends", but I'd have to say, they're a popular source of free feed, I can pretty much rely on throughout the season. So they're kind of like a friend.
When everything else has bitten the proverbial dust, I can be certain to find these somewhere in the garden. The only annoyance they really give me, is in autumn, when they've gotten away on me. I'll know it, as soon as every piece of clothing comes into the house, with scratchy seeds attached.
They annoy me, only because I could have turned them into eggs sooner. As I patiently pluck them out of my clothes, I think of attacking them better next year, to feed my hungry egg makers. I know I'll never eradicate these weeds, and I won't be poisoning them, so relegated to egg making, it is.
There's only one weed I didn't get to take pictures of, because it takes longer to produce fruit. The chickens eat the fruits, not the leafs, as those are quite toxic. They are the black fruit of the deadly nightshade. One bush can produce a lot of fruit and it's not just the chickens that love them. Every native bird here, comes to rely on the self-seeding deadly nightshades.
I pluck the fruit and throw in handfuls. The chickens go nuts over them. Be sure to only pick the black berries however, as any with green, have higher doses of solanine present, and overdosing can be unpleasant. All the weeds I've outlined here however, are all naturally occurring. I haven't done anything, but glean the harvest. These weeds can also be fed to guinea pigs (except the deadly nightshade) who often get several helpings from me, a day.
So next time you think a weed is getting in your way, turn it into a free source of chicken food, instead.